1. Field of the Invention
The present invention pertains to fiber optic systems utilizing erbium-doped fiber amplifiers (EDFA). More particularly, the invention relates to a heating system for inclusion into an existing EDFA optic system for heating an erbium fiber to maintain optimal EDFA performance.
2. Description of the Related Art
The use of EDFAs in fiber optic communication systems is known. Such EDFAs are used in conjunction with a pump laser which provides optical power at a select frequency for EDFA operation. In existing systems, e.g. systems that are already in place, the pump laser and a thermo-electric cooler (TEC) are produced on a common substrate or chip and the TEC is used to regulate the laser temperature by heating and cooling the pump laser.
One such existing system 10 is depicted in FIG. 1. As there shown, a pump laser 12 is thermally connected to a TEC 14 which, depending on certain conditions, provides heat to and removes heat from the pump laser. The laser temperature is determined using a temperature sensor such as a thermistor 16 connected to a control circuit 18. It is desirable for pump laser 12 to operate below a maximum temperature value and above a minimum temperature value. The control circuit 18 measures the voltage drop across the thermistor 16 and, depending on the measurement, generates a control signal to cause heating or cooling by the TEC. For this purpose, a switching means 20 is connected between a positive voltage supply (V+) and a negative voltage supply (V-). When a temperature below the minimum desired temperature is measured, the control circuit activates the switching means 20 to provide a positive current to the TEC for heating the TEC and, consequently, the laser 12. If, on the other hand, a temperature above the maximum desired temperature is measured from thermistor 16, control circuit 18 causes switching means 20 to provide a negative or oppositely-flowing current to the TEC, thereby causing the TEC to cool the laser.
More specifically, and as understood by those having ordinary skill in the art, the temperature sensor 16 is influenced by heat from the EDFA system environment, by heat produced in normal operation by the pump laser 12, and by heat removed (or added) by the TEC 14. When the temperature of the environment lies above a certain minimum (environment) temperature, the heat from the environment and pump laser 12 causes the temperature of the temperature sensor 16 to rise above a certain minimum (temperature sensor) temperature. The control circuit 18 responds to the temperature of the temperature sensor 16 by activating the switching means 20 to provide an electric current to the TEC 14 to cause heat to be removed from the region in which the temperature sensor 16 and pump laser 12 are disposed. When the temperature of the environment falls below a certain minimum (environment) temperature, the heat from the pump laser 12 is insufficient to maintain the temperature of the temperature sensor 16 above a certain minimum (temperature sensor) temperature. The control circuit 18 responds to the temperature of the temperature sensor 16 by activating the switching means 20 to provide electric current to the TEC 14 to cause heat to be added to the region in which the temperature sensor 16 and pump laser 12 are disposed. Thus it is to be understood that in the prior art, a temperature sensor 16 disposed in thermal contact with pump laser 12 and TEC 14 can be used in conjunction with a control circuit 18 to determine whether the environment temperature lies above a certain maximum or below a certain minimum temperature.
It has been discovered that the gain spectrum of erbium fiber varies with temperature. Further, it is desirable to use a heating system to control the temperature of the erbium fiber, (e.g. to maintain the fiber temperature within a specified range), thereby controlling the variation in the gain spectrum. This can be accomplished by designing a heating system where the erbium fiber and a temperature sensor are both in thermal contact with a heating element. A control circuit, such as control circuit 18 of FIG. 1 can be used to measure the voltage drop across the temperature sensor and, when the measured voltage exceeds a certain value, the control circuit activates a switch which provides electric current to the heating element for heating the erbium fiber to a temperature above a selected minimum temperature.
In the prior art systems developed following the discovery that it is desirable to control the erbium fiber temperature in EDFAs, it is known that a heating system can be incorporated in the EDFA design. In particular, this requires the inclusion in the EDFA system of a heating element, a temperature sensor, a control circuit to monitor the temperature sensor and a switch to provide electric current to the heating element. However, in EDFA systems designed and built prior to the discovery of the desirability of controlling the erbium fiber temperature, no provisions exist for an erbium fiber heating system. Providing control of the erbium fiber temperature for such existing systems would require extensive redesign of control circuitry, including the addition of a temperature sensor and a supply of electric current for a heating element.
Accordingly, it is desirable to provide an easily implemented upgrade to an existing fiber optic EDFA system for regulating fiber temperature without the need for extensive redesign of existing control circuitry.